Aromatic polyester polyols are commonly used intermediates for the manufacture of polyurethane products, including flexible and rigid foams, polyisocyanurate foams, coatings, sealants, adhesives, and elastomers. The aromatic content of these polyols contributes to strength, stiffness, and thermal stability of the urethane product.
Commonly, the aromatic polyester polyol is made by condensing aromatic diacids, diesters, or anhydrides (e.g., terephthalic acid, dimethyl terephthalate) with glycols such as ethylene glycol, propylene glycol, diethylene glycol, or the like. These starting materials usually derive exclusively from petrochemical sources.
As companies increasingly seek to offer products with improved sustainability, the availability of intermediates produced from bio-renewable and/or recycled materials becomes more leveraging. However, there remains a need for these products to deliver equal or better performance than their traditional petroleum-based alternatives at a comparable price point.
Bio-renewable content alone can be misleading as an indicator of “green” chemistry. For example, when a food source such as corn is needed to provide the bio-renewable content, there are clear trade-offs between feeding people and providing them with performance-based chemical products. Additionally, the chemical or biochemical transformations needed to convert sugars or other bio-friendly feeds to useful chemical intermediates such as polyols can consume more natural resources and energy and can release more greenhouse gases and pollutants into the environment than their petro-based alternatives in the effort to achieve “green” status.
Waste thermoplastic polyesters, including waste polyethylene terephthalate (PET) streams (e.g., from plastic beverage containers), provide an abundant source of raw material for making new polymers. Usually, when PET is recycled, it is used to make new PET beverage bottles, PET fiber, or it is chemically transformed to produce polybutylene terephthalate (PBT). Other recycled raw materials are also available. For example, recycled propylene glycol is available from aircraft or RV deicing and other operations.
Urethane formulators demand polyols that meet required specifications for color, clarity, hydroxyl number, functionality, acid number, viscosity, particulates, and other properties. These specifications will vary and depend on the type of urethane application. For instance, rigid foams generally require polyols with higher hydroxyl number than the polyols used to make flexible foams.
Polyols suitable for use in making urethanes have proven difficult to manufacture from recycled materials, including recycled polyethylene terephthalate (rPET). Many references describe digestion of rPET with glycols (also called “glycolysis”), usually in the presence of a catalyst such as zinc or titanium. Digestion converts the polymer to a mixture of glycols and low-molecular-weight PET oligomers. Although such mixtures have desirably low viscosities, they often have high hydroxyl numbers or high levels of free glycols. Frequently, the target product is a purified bis(hydroxyalkyl) terephthalate (see, e.g., U.S. Pat. Nos. 6,630,601, 6,642,350, and 7,192,988) or terephthalic acid (see, e.g., U.S. Pat. No. 5,502,247). Some of the efforts to use glycolysis product mixtures for urethane manufacture are described in a review article by D. Paszun and T. Spychaj (Ind. Eng. Chem. Res. 36 (1997) 1373.
Most frequently, ethylene glycol is used as the glycol reactant for glycolysis. This is sensible because it minimizes the possible reaction products. Usually, the glycolysis is performed under conditions effective to generate bis(hydroxyethyl) terephthalate (“BHET”), although sometimes the goal is to recover pure terephthalic acid. When ethylene glycol is used as a reactant, the glycolysis product is typically a crystalline or waxy solid at room temperature. Such materials are less than ideal for use as polyol intermediates because they must be processed at elevated temperatures. Polyols are desirably free-flowing liquids at or close to room temperature.
Gel permeation chromatography has been used to identify components of mixtures produced by glycolysis of rPET, although the reported results are not necessarily in agreement about the proportion of terephthalate monomer, dimer, and trimer products generated. For instance, Vaidya et al. (J. Appl. Polym. Sci. 34 (1987) 235) teach that depolymerization of PET waste using 37.5, 50, or 62.5 wt. % of propylene glycol and zinc acetate catalyst (200° C., 8 h) provides a mixture in which the major fractions are identified as monomeric, i.e., species that have a single terephthalate unit (BHET, bis(hydroxypropyl) terephthalate (“BHPT”), and the mixed bis(hydroxyalkyl) terephthalate monomer). In two papers (J. Appl. Polym. Sci. 92 (2004) 3040; 105 (2007) 1802), Saravari et al. describe the synthesis of urethane oils from palm oil and waste PET bottles. In an initial step, the PET is depolymerized using propylene glycol (62.5 wt. %) and zinc acetate (190° C., 6 h). The GPC chromatogram of the glycolized product from the 2004 paper (FIG. 1, p. 3042) shows what appears to be a mixture of dimer (458 mol. wt.) and trimer (844 mol. wt.), although the text suggests that monomers are also present. A similar chromatogram in the 2007 paper (FIG. 2, p. 1804) reveals what appears to be roughly a 1:3:1 mixture of monomer (249 mol. wt.), dimer (553 mol. wt.), and trimer (831 mol. wt.), prepared by the same method used earlier, and the text is consistent with this interpretation. Further, Ikladious (J. Elast. Plast. 32 (2000) 140) describes waste PET depolymerization with propylene glycol (40-60 wt. %) and zinc acetate (200° C., 10 h). GPC chromatograms (FIGS. 1-3) indicate two predominant products (473 and 759 mol. wt.), which the authors attribute to dimer and trimer products, respectively. Using more propylene glycol tilts the product mixture in the direction of more dimer. Interestingly, however, despite similar process conditions compared with those used by the other authors, the GPC trace shows no monomers.
Because rPET is frequently colored with dyes (e.g., green PET used for soda bottles), activated carbon treatment has been used to reduce the color level during the preparation of BHET or terephthalic acid. For instance, U.S. Pat. No. 7,192,988 teaches a two-stage process for color removal. Recycled PET is depolymerized to give BHET, which is treated with activated carbon to remove some color. The remaining dye is removed by extraction with water, alcohol, or glycol to recover purified BHET, which can be used as a monomer for making PET. For additional examples of activated carbon treatment, see U.S. Pat. Nos. 5,504,121; 5,602,187; 6,630,601; and 6,642,350. Extraction methods have also been used to remove color from glycolized, colored rPET (see, e.g., U.S. Publ. No. 2012/0149791). Thus, there has been limited activity in converting rPET to useful polyol compositions and even less interest in removing color from such compositions, as prior color removal efforts tend to focus on purified monomer compositions.
Improved polyols are needed. In particular, the urethane industry needs sustainable polyols based in substantial part on recycled polymers such as the practically unlimited supply of recycled polyethylene terephthalate. Polyols with high recycle content that satisfy the demanding color, clarity, viscosity, functionality, and hydroxyl content requirements of polyurethane formulators would be valuable.